The invention relates to improvements in methods of, and in apparatus for, transmitting torque from a prime mover to one or more driven units, for example, for transmitting torque from a rotary output element (such as a crankshaft or a camshaft) of the engine to one or more wheels of a motor vehicle. More particularly, the invention relates to improvements in methods of, and in apparatus for, transmitting torque by way of a hydrokinetic torque converter which is equipped with a lockup clutch or bypass clutch. Still more particularly, the invention relates to improvements in methods of, and in apparatus for, transmitting torque by way of a hydrokinetic torque converter which can transmit torque by way of a turbine and/or by way of a slipping lockup clutch constructed and assembled to operate in parallel with the turbine. The invention also relates to a method of, and to means for, regulating the operation of a slipping lockup clutch for the purpose of ensuring that the magnitude of torque which the clutch can transmit is a function of then prevailing operating conditions.
In accordance with a presently preferred embodiment, the method of the instant invention can be practiced to regulate the operation of an apparatus which can be installed in the power train of a motor vehicle to transmit torque between the output element of the engine and the input element of an automatic transmission, and wherein the apparatus employs (a) a hydrokinetic torque converter having a turbine and a lockup clutch (such as a friction clutch) operating in parallel with the turbine, (b) means for monitoring one or more variable parameters and for generating and memorizing corresponding signals, and (c) means (such as a central computer or processor) for evaluating, processing and applying the signals to regulate the slippage of, as well as the transmission of, torque by the lockup clutch.
As a rule, a hydrokinetic torque converter which can be utilized in the novel apparatus and/or in accordance with the novel method comprises a pump, a turbine, a stator and a housing or cover which is driven by the rotary output element of a prime mover (such as the engine of a motor vehicle) and transmits torque to the pump. The cover is coaxial with the pump and with the turbine and defines a chamber which accommodates the turbine as well as a lockup clutch. The lockup clutch can transmit torque from the cover directly to the turbine or to a part which is driven by the turbine, and such clutch can constitute or include a friction clutch having a first friction surface on a substantially radially extending portion of the cover and a second friction surface provided on a piston which is movable in the cover in the axial direction of the turbine to move its friction surface into or away from frictional engagement with the first friction surface so that the magnitude of torque which the clutch can transmit depends on the extent of frictional engagement between the first and second surfaces. The second friction surface is normally provided on a radially outer portion of the piston, and the radially inner portion of such piston can transmit torque directly to the turbine or to a part (e.g., the rotary input element of a transmission which receives torque from the turbine or a hub which is of one piece with or is separably connected to the turbine and is also non-rotatably coupled to the input element) which receives torque from the turbine when the two friction surfaces of the lockup clutch are free to slide relative to each other.
Apparatus of the above-outlined character are disclosed, for example, in published German patent application No. 31 30 871, in U.S. Pat. No. 5,029,087 and in U.S. Pat. No. 4,577,737.
It is also known to regulate the operation of the lockup clutch in a conventional hydrokinetic torque converter by selectively varying the pressure in an internal chamber of the torque converter or by selectively varying the pressure differential between bodies of fluid in compartments at opposite sides of the aforementioned piston. The purpose of such regulation is to select those stages or phases of operation of the apparatus when the lockup clutch is called upon to transmit torque directly from the cover (i.e., from the output element of the prime mover) to the part or parts which are to receive torque from the turbine when the lockup clutch is not in use.
The aforementioned published German patent application No. 31 30 871 discloses a method which includes monitoring the slippage between the input and output members of the lockup clutch, comparing the thus ascertained values with preselected values of slip, and adjusting the operation of the lockup clutch when the monitored values depart from the preselected values. The German patent application proposes to adjust the operation of the lockup clutch by varying the pressure on a body of fluid in a chamber at one side of the axially movable piston of the lockup clutch until the difference between the RPM of the input member and the RPM of the output member of the clutch reaches a desired value, at least within a relatively low RPM range of the output element of the prime mover. In other words, the method which is proposed in the published German patent application is based on the well-known principle of regulating the slippage between the input and output members of the lockup clutch when the actual slippage departs from a predetermined slippage.
The disclosure of the U.S. Pat. No. 5,029,087 is analogous to that of the aforediscussed published German patent application No. 31 30 871, i.e., the U.S. reference also discloses a method of monitoring the slippage of the lockup clutch in the cover of a hydrokinetic torque converter, comparing the thus ascertained slippage with preselected values, and regulating the pressure of a body of fluid in a compartment of the torque converter in a sense to eliminate the differences between the actual slippage and the desired slippage of the lockup clutch. The patent further proposes to regulate the slippage of the lockup clutch in the aforedescribed manner while the RPM of the output element of the prime mover is within a relatively low range of revolutions per minute.
The disclosure in the U.S. Pat. No. 4,577,737 is also analogous to that of the published German patent application No. 31 30 871.
The proposals to regulate the operation of a lockup clutch in a manner as disclosed in the aforediscussed publications have met with limited commercial success or no commercial success at all. The reason for such absence of acceptance is believed to be that the aforementioned patents and the aforementioned patent application propose to regulate the slippage of the input and output members of a lockup clutch relative to each other while the RPM of the prime mover driving the cover of the hydrokinetic torque converter is relatively low, namely immediately above the idling speed of the prime mover. If the prime mover is the engine of a motor vehicle, the vehicle is likely to be operated, primarily or even exclusively, in such a way that the RPM of the output element of the engine is within the aforementioned relatively low range of rotational speeds. This means that, due to slippage of the input and output members of the lockup clutch in order to prevent the transmission of oscillations to the driven unit or units (such as an automatic transmission), the energy requirements (i.e., the fuel consumption) of the motor vehicle are increased accordingly. Furthermore, the slippage which is to take place while the RPM of the output element of the engine is relatively low (i.e., immediately or closely above the idling RPM) cannot be selected at will or with a requisite degree of accuracy because the operating parameters and operating conditions which respectively develop and take place at such low RPM of the output element of the engine cannot be relied upon to select the fluid pressure which is necessary to effect a slippage-free engagement of the lockup clutch. One of the reasons is that the pressure of fluid in the cover of the torque converter is low when the transmitted torque is relatively small and such low pressure of fluid cannot be regulated with the degree of accuracy which is needed to ensure that the slippage of the input and output members of the lockup clutch will vary with a degree of predictability which is needed to guarantee that the magnitude of torque being transmitted from the output element of the engine to one or more driven units in the power train of a motor vehicle will correspond to optimum torque for the prevailing operating conditions. It has been ascertained that, since the pressure of the fluid in the cover of the torque converter is relatively low when the magnitude if the transmitted torque is small, even minor fluctuations of fluid pressure are likely to entail pronounced variations of slippage between the driving and driven members of the lockup clutch. Furthermore, it is necessary to take into consideration the hysteresis of the valve or valves which are utilized in such torque converters to regulate the pressure of the fluid in the interior of the cover (e.g., the friction between the cylinder and the piston of a valve which forms part of the controls for the torque converter), and such hysteresis renders it necessary to maintain a certain level of fluid pressure in order to account for the hysteresis. Otherwise stated, the accuracy of regulation of the torque which is being transmitted by the lockup clutch in conventional hydrokinetic torque converters decreases in response to a reduction of the magnitude of torque to be transmitted from a prime mover to one or more driven units, e.g., from the combustion engine to the transmission in a motor vehicle.
Another drawback of heretofore known proposals to regulate the operation of a lockup clutch or bypass clutch in a hydrokinetic torque converter is that, when the RPM of the output element of the engine is relatively low and the load upon the power train of the vehicle is also low (this takes place quite frequently when the RPM of the output element of the engine is not much higher than the idling RPM), low-amplitude fluctuations of transmitted torque often result in short-lasting adherence to one another of abutting friction surfaces of the input and output members of the lockup clutch at a time when the lockup clutch is supposed to slip. The intervals of adherence alternate with intervals of slippage, and such alternating slippage and adherence entail the generation of pronounced rattling and buzzing noises in the power train of a motor vehicle. Moreover, alternating intervals of adherence and slippage often initiate abrupt changes of the torque which is being transmitted to the input element of a transmission in the power train of a motor vehicle. The only heretofore known solution for such problems is to increase the slippage between the input and output members of the lockup clutch which, in turn, entails highly increased energy requirements for the engine.
Still another drawback of heretofore known proposals to regulate the slippage of a lockup clutch in a hydrokinetic torque converter in the power train between the prime mover and the transmission of a motor vehicle is that, when the RPM of the output element is relatively low (e.g., within a range immediately above the idling RPM), i.e., when the power train is under a mere partial load, the torque which is to be transmitted from the lockup clutch to the driven input element of a transmission or another driven unit of the motor vehicle can be reduced to the required value only with a considerable outlay for regulating equipment. The reason is that the magnitude of the torque to be transmitted under such circumstances is not dependent solely upon the clutch engaging force but also depends on the characteristics of the friction surfaces of the input and output members of the lockup clutch. Such characteristics of the friction surfaces, in turn, are a function of a number of different parameters including the temperature of the input and/or the output member, the RPM at which the friction surfaces are to slip relative to one another, the characteristics of the fluid (e.g., oil) in the cover of the hydrokinetic torque converter and/or certain other factors. Therefore, the characteristics of the friction surfaces are likely to fluctuate within a very wide range and, consequently, the means for regulating the slippage must be designed to take into consideration and to compensate for the influence of at least some if not all of the above-enumerated parameters. This is proposed to be accomplished by selecting a relatively high RPM at which the friction surfaces of the input and output members of the lockup clutch begin to slip relative to each other, namely to adhere to a relatively high minimum RPM at which the lockup clutch begins to slip. This is intended to ensure that the RPM at which the lockup clutch will begin to slip is sufficiently high to prevent the transmission of fluctuations of torque of the output element of the prime mover to the input element or elements of one or more units receiving torque from the turbine of the hydrokinetic torque converter or from the lockup clutch.